This invention is generally directed to conductive composite particles and processes for the preparation thereof, and more specifically, the present invention relates to the preparation of small conductive polymeric composite particles of, for example, from about 0.05 to about 5 and preferably from about 0.1 to about 1 micron in diameter, each comprising a polymer and a conductive filler distributed, preferably evenly throughout the polymer matrix of the particle composite comprised of polymer and conductive filler. The present invention also relates to processes for the preparation of polymeric composite particles by a modified bulk polymerization process. In embodiments, the process of the present invention comprises the preparation of conductive polymeric particles optionally containing a conductive filler in an amount of from about 1 to about 50 and preferably from about 5 to about 20 weight percent distributed substantially throughout the polymer matrix of the particles, and which particles can be selected as carrier powder coatings, and wherein the carriers can be selected for xerographic imaging and printing processes. In embodiments, the process of the present invention comprises the preparation of conductive polymeric composite particles with an average volume particle size diameter of from between about 0.05 micron to about 5 microns. The conductivity of the generated submicron polymeric composite particles can be modified by, for example, varying the weight percent of conductive filler component present in effective amounts of preferably, for example, from between about 5 to about 20 weight percent, and also by varying the composition of the conductive filler component. Thus, in embodiments conductive submicron polymeric composite particles with a conductivity of from between about 10.sup.-10 (ohm-cm).sup.-1 to about 10.sup.-1 (ohm-cm).sup.-1 can be prepared. In embodiments, the particles with an average volume diameter of from about 0.05 to about 5 microns are comprised of polymer and a conductive filler or additive like carbon black distributed evenly throughout the polymer matrix of the composite product, and which product can be obtained by a semisuspension polymerization method using a modified bulk polymerization in which at least one monomer is mixed with a conductive filler, a redox initiator pair comprising an oxidizing agent, such as a free radical initiator, and a suitable reducing agent, one or more other free radical initiator polymerization initiators, and optionally a chain transfer component; effecting bulk polymerization by heating until from about 4 to about 50 weight percent of the monomer has been polymerized and one or both components of the redox initiator pair have been consumed; dispersing the resulting mixture in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 5 microns in water; polymerizing the resulting suspension by heating; and optionally subsequently washing and drying the polymer product; and wherein the aforementioned bulk polymerization is accomplished at about room temperature, or a temperature of from about zero to about 70.degree. C. Optionally, the conductive filler can be added at the end of the bulk polymerization and mixed into the organic phase using, for example, a high shear mixer.
Metals, such as carrier cores, are conductive or semiconductive materials, and the polymeric materials used to coat the surface of metals are usually insulating. Therefore, carrier particles coated completely with polymer or a mixture of polymers can lose their conductivity and become insulating. Although this is desired for some applications, such as for conductive magnetic brush systems (CMB), the carrier particles should be conductive. Since the carrier polymer coating can be utilized to control carrier tribo, a conductive carrier coating is needed to design carriers with the desired conductivity and triboelectrical properties. Conductive polymers are very costly, and are not considered suitable for preparing low cost, for example less than $5/pound, coating, thus a conductive polymer composite comprising a low cost polymer and a conductive filler, such as conductive carbon black, is considered a more suitable alternative.
A polymer composite coating of metal materials, such as carrier beads, is known and can generally be obtained by two general approaches, solution and powder coating. Solution coating of carriers with a polymer composite solution comprised of a polymer, a conductive filler and solvent can be utilized to prepare conductive carrier, however, trapping of solvent in the solution coating can adversely interfere with the use of coated materials, for example the residual solvent trapped in the carrier coating reduces the carrier life, and the release of solvent in the developer housing can cause other problems related to the harmful effects of absorbed solvent to various copying machine parts and the toxicity of solvent. Moreover, the solvent recovery operation involved in the solution coating processes is costly. The powder coating of metal surfaces can eliminate the need for solvent, and therefore, many of the problems associated with solution coating; however, it requires a polymer powder with a very small size, for example less than 1 to 5 microns. Although polymer powders are available for carrier powder coating, submicron or micron-sized polymer composite particles containing conductive filler to prepare conductive coated carriers that maintain their triboelectrical characteristics for extended time periods exceeding, for example, 200,000 images, and which possess the other advantages illustrated herein are desired.
In copending application U.S. Ser. No. 225,855, now U.S. Pat. No. 5,487,847, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of conductive polymeric particles with linear and crosslinked portions, which comprises mixing a monomer with at least one conductive filler, solvent, at least one polymerization initiator, and a chain transfer component; effecting solution polymerization by heating until from about 80 to about 100 weight percent of the monomer has been polymerized; drying the mixture by removing the solvent to yield an intimate blend of polymer with conductive filler; dispersing the aforementioned blend of polymer and conductive filler in at least one monomer with at least one polymerization initiator, a crosslinking agent and a chain transfer agent to form an organic phase; dispersing the resulting organic phase mixture in water containing a stabilizing component to obtain a suspension of particles with an average volume diameter of from about 0.05 to about 5 microns in water; polymerizing the resulting suspension by heating; and subsequently optionally washing and drying the polymeric product. The invention of the present application in embodiments eliminates the need, for example, to conduct the solution polymerization and subsequently remove the solvent such as toluene. The invention of the present application offers an economical process requiring less process time, less capital investment and lower processing costs as compared to the process disclosed in the above mentioned copending application.
Semisuspension polymerization processes are known, reference U.S. Pat. No. 5,236,629, the disclosure of which is totally incorporated herein by reference. The '629 patent describes a process for the preparation of conductive submicron polymeric particles, which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has been polymerized; terminating polymerization by cooling the partially polymerized monomer; adding thereto from about 1 to about 50 weight percent of a conductive filler, or conductive fillers, followed by mixing thereof; dispersing the aforementioned mixture of conductive filler or fillers, and partially polymerized product in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 1 micron in water; polymerizing the resulting suspension by heating; and subsequently washing and drying the product. It is indicated in the '629 patent that the viscosity of the organic phase at the end of the bulk polymerization is important in determining the final particle size and also in determining the quality of the pigment or carbon black dispersion in the particles. It is, therefore, important to have good control of the conversion at the end of the bulk polymerization to ensure a reproducible process. While it is difficult to establish adequate control of a bulk polymerization of monomers in the absence of conductive additives, it is much more difficult to control the conversion in the bulk polymerization when conductive additives are present since many of these materials, for example carbon blacks, are known to contain functional surface groups that act as polymerization inhibitors. When conducting a bulk polymerization with such inhibitors present, there is a greater variability in the polymerization rate than in the absence of these materials and consequently process reproducibility is lower. Consequently, there exists a need for a process that allows a reproducible process to be achieved during bulk polymerization, particularly in the presence of conductive additives. The invention of the present application in embodiments selects a free radical redox initiator system selected to provide low temperature reaction conditions, where low temperature refers to the range of from about 0.degree. to about 70.degree. C., and more preferably from about 0.degree. to about 30.degree. C., a reproducible conversion in the bulk polymerization of from about 4 to about 50 percent, which can be varied by controlling the ratio of the redox initiator pair by, for example, varying the ratio of benzoyl peroxide to dimethylaniline. With conventional bulk polymerization processes the monomer conversion increases with time as illustrated, for example, in FIG. 1.
The kinetics of the bulk polymerization of the present invention are illustrated in FIG. 2.
In conventional bulk polymerization, the conversion increases approximately linearly with time until the onset of the gel effect at which point it increases rapidly. Achieving a reproducible process requires that the polymerization be terminated at almost exactly the same time, and that the polymerization kinetics be identical each process run. However, in the presence of conductive additives containing functional groups that act as inhibitors or retarders, the kinetics will usually vary from FIG. 3.
Consequently, there will be large, that is for example the standard deviation of the conversion will exceed for instance 0.02, variation in the conversion at the end of the bulk polymerization. The invention herein eliminates this difficulty by establishing kinetics in which the conversion increases to a certain value and then stops increasing (FIG. 2). In this process, there is no need to terminate the bulk polymerization at a specific time to achieve a desired conversion. The same desired conversion is achieved whenever the process is operated by allowing the bulk polymerization to function for a sufficiently long period of time, for example from about 1 to about 3 hours. Other advantages of the modified bulk polymerization processes of the present invention include elimination of an uncontrolled exothermic reaction. An uncontrolled exothermic reaction could result in a conventional bulk process if, for example, there was a loss of cooling to the reactor or if mixing was terminated leading to reduced heat transfer. These conditions would not cause an uncontrolled exothermic reaction with the modified bulk polymerization process since, for example, the redox initiator system is chosen to active at lower temperature and to be consumed at the end of the bulk polymerization. Once the desired conversion level is reached, the redox initiator system has been consumed, and therefore, further polymerization can only proceed when the mixture has been heated to a higher temperature to activate the conventional free radical initiators. A further advantage of this invention is a reduction of the formation of free conductive filler particles contaminating the product. Free conductive filler particles arise when the conversion at the end of the bulk polymerization is too low and the conductive filler particles then diffuse out of the polymerizing polymer particle during the suspension polymerization step.
The redox initiator pair system of the present invention comprises an oxidizing agent which is also a free radical initiator, for example a peroxide, and a suitable reducing agent, for example dimethylaniline. The reducing agent could also be an additive, such as carbon black, containing surface groups that act as reducing agents. By varying the ratio of benzoyl peroxide to dimethylaniline, the limiting conversion of a methyl methacrylate polymerization can be varied from, for example, 0.04 to 0.5; and also the ratio and concentration of the oxidizing and reducing agents forming the redox initiator pair permits the conversion at the end of the bulk polymerization to be controlled.
The preparation of polymeric particles for powder coatings can be accomplished, for example, by three methods, namely grinding or attrition, precipitation and in situ particle polymerization. Grinding or attrition, especially fluid energy milling, of large polymeric particles or polymeric composite particles containing fillers to the size needed for powder coating, for example less than 1 to 5 microns, is often not desirable both from an economic and functional viewpoint. These materials are difficult to grind and, therefore, grinding or attrition of required materials for coating with present milling equipment is very costly due to very low processing yield, for example in the range of 5 to 10 weight percent. Precipitation process can also be used to prepare polymeric/polymeric composite particles. In one process, the polymer solution is heated to above its melting temperature and then cooled to form particles. In another process, the polymer solution is precipitated using a nonsolvent or the polymer solution is spray dried to obtain polymeric/polymeric composite particles. With these precipitation processes, it has been difficult to achieve low cost, pure polymer, that is, for example, with no or substantially no impurities such as solvents or precipitants in the resulting polymer particles. It is also difficult to obtain particles with small particle size and narrow particle size distribution. Further, it can be difficult to control filler distribution throughout each particle's polymer matrix. In the in situ particle polymerization process, polymer particles are prepared by suspension dispersion, emulsion and semisuspension polymerization. Suspension polymerization can be utilized to prepare polymer particles and polymeric composite particles containing, for example, a conductive filler. However, this process does not, for example, effectively enable particles with a size of less than 5 microns. Although emulsion and dispersion polymerization can be utilized to prepare polymeric particles of a small size, for example less than 5 microns, these processes wherein particle formation is achieved by nucleation and growth do not, it is believed, enable synthesis of particles containing fillers such as conductive fillers.
There is disclosed in U.S. Pat. No. 4,908,665 a developing roller or developer carrier comprised of a core shaft, a rubber layer and a resin coating layer on the surface of the rubber containing conductive fillers for a one component developer. It is indicated in the '665 patent that a conductive developing roller can eliminate variation of the image characteristic due to the absorption of moisture for one component development. This patent thus describes a developing roller for one component developer. U.S. Pat. No. 4,590,141 discloses carrier particles for two component developer coated with a layer of silicon polymer using fluidized bed solution coating. U.S. Pat. No. 4,562,136 discloses a two component dry type developer of carrier particles coated with a silicon resin containing a monoazo metal complex charging. The two component carriers described in the above two patents are insulating and are not believed to be conductive. There is disclosed in U.S. Pat. No. 4,912,005 a conductive carrier composition coated with a layer of resin containing a conductive particle by solution coating. Residual solvent trapped in the aforementioned coated layer adversely effects the maintainability of carrier electrical properties for an extended time period.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles for fluidized bed powder coating are prepared by dispersing the polymer in a liquid, which is heated to above the polymer melting point and stirred causing the polymer particles to form. The particles are then cooled below their melting point and recovered. However, this process does not, for example, enable particles with a size of below 50 microns in average volume diameter.
Also, the suspension polymerization of monomer is known for the formation of polymer/polymeric composite particles generally in a size range of about 200 microns and higher. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also, disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets.
Disclosed in U.S. Pat. No. 4,835,084 is a method for preparing pigmented particles wherein high concentration of silica powder is used in the aqueous phase to prevent coalescence of the particles. There is also disclosed in U.S. Pat. No. 4,833,060 a process for the preparation of pigmented particles by dissolving polymer in monomer and dispersing in the aqueous phase containing silica powder to prevent coalescence of the particles. However, the silica powder used in both U.S. Pat. Nos. '084 and '060 should be removed using KOH, which is costly, and residual KOH and silica materials remaining on the surface adversely affects the charging properties of particles. There is also disclosed in U.S. Pat. No. 3,954,898 a two step polymerization process for the preparation of a thermosetting finished powder. However, this process does not enable synthesis of particles with a size less than about 100 microns.
As a result of a patentability search for U.S. Pat. No. 5,043,404, there were located U.S. Patents 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; 4,680,200 and 4,702,988, which illustrate emulsion polymerization. It is known that submicron polymeric particles can be synthesized by emulsion polymerization. However, synthesis of submicron polymeric particles by emulsion polymerization requires a high concentration of emulsifier which remains in the final product and renders it humidity sensitive. Therefore, emulsion polymerization does not enable preparation of clean submicron polymeric particles which are insensitive to humidity. Moreover, in emulsion polymerization particle formation is controlled by diffusion of monomer from monomer droplet through a water phase into the growing particles. This mechanism, which is characteristic of emulsion polymerization, does not allow inclusion of conductive fillers in the polymeric particles. Furthermore, it is known that the addition of conductive fillers into emulsion, dispersion or suspension polymerization systems causes severe inhibition which stops or reduces the rate of polymerization significantly.
Disclosed in the aforementioned U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference, is a semisuspension polymerization process for the preparation of small polymeric particles which are comprised of a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component which are bulk polymerized until partial polymerization is accomplished. The resulting partially polymerized monomer or comonomers is dispersed in water containing a stabilizer component with, for example, a high shear mixer, then the resulting suspension polymerized, followed by washing and drying the submicron polymeric particles. U.S. Pat. No. 5,236,629 discloses a process for the preparation of conductive submicron polymeric particles which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has been polymerized; terminating polymerization by cooling the partially polymerized monomer; adding thereto from about 1 to about 50 weight percent of a conductive filler, or conductive fillers, followed by mixing thereof; dispersing the aforementioned mixture of conductive filler or fillers, and partially polymerized product in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 1 micron in water; polymerizing the resulting suspension by heating; and subsequently washing and drying the product.
The modified bulk semisuspension polymerization process described in the present application offers a different process with significant improvements over the process disclosed in U.S. Pat. No. 5,236,629. These advantages include (1) superior process reproducibility of the bulk polymerization step, which in turn leads to superior and improved control of the final product particle size and conductive filler dispersion; (2) an inherently safer process with a minimal risk of a runaway exothermic reaction occurring during the bulk polymerization; and (3) fewer potential problems with free conductive filler particles (caused by low conversion during the bulk polymerization step) contaminating the final product and causing, for example, problems with contamination in the developer housing when the particles are coated on carrier cores. The modified bulk semisuspension polymerization processes of the present invention permit the preparation of low cost, clean, and dry submicron conductive polymeric particles that can be selected as carrier powder coatings.